1. Field of the Invention
The present invention relates generally to a power supply apparatus for resistance welding, and more particularly to a polarity switching power supply apparatus designed to supply a welding current of AC waveform to workpieces.
2. Description of the Related Arts
A current-supplying system flowing AC waveform welding current is prevailing of late in two-point simultaneous joining resistance welding (series welding) which is mainly applied to workpieces in the form of small metal pieces such as electronic components. Reference is first made to FIGS. 9A to 9D which illustrates an example of the series welding based on such a current-supplying system.
In the series welding of FIGS. 9A to 9D, a pair of welding electrodes 10 and 12 abut against one surfaces of workpieces (W1 and W2) at spaced-apart positions and come into pressure contact with the workpieces (W1 and W2) by a pressure force from a pressure mechanism not shown. In this series welding there alternate a single positive current supplying in which the welding current flows in a positive direction and a single negative current supplying in which the welding current flows in a negative direction.
In the former-half positive current supplying (period TA), a welding current IW flows in a positive direction through a path from the welding electrode 10 through the workpiece W1, a first weld point Pa, the workpiece W2, a second weld point Pb and again the workpiece W1 to the welding electrode 12 as depicted in FIG. 9A. At that time, at the weld point Pa the welding current IW flows from the workpiece W1 toward the workpiece W2 whereas at the weld point Pb the welding current IW flows from the workpiece W2 toward the workpiece W1. As a result there arises for instance heat-absorbing Peltier effect at the first weld point Pa, with heat-generating Peltier effect at the second weld point Pb. Thus, in this positive current supplying, as shown in FIG. 9B a nugget Nb at the second weld point Pb can grow at a greater growth rate than a nugget Na at the first weld point Pa. At this point of time, the difference in size between the two nuggets Na and Nb may depend on e.g., material and thickness of the workpieces (W1 and W2), the length of the weld period TA and current value of the welding current IW.
In the latter-half positive current supplying (period TB), the welding current IW flows in a negative direction through a path from the welding electrode 12 through the workpiece W1, the second weld point Pb, the workpiece W2, the first weld point Pa and again the workpiece W1 to the welding electrode 10 as depicted in FIG. 9C. At that time, at the weld point Pa the welding current IW flows from the workpiece W2 toward the workpiece W1 whereas at the weld point Pb the welding current IW flows from the workpiece W1 toward the workpiece W2. As a result there now arises heat-generating Peltier effect at the first weld point Pa, with heat-absorbing Peltier effect at the second weld point Pb. For this reason, in this negative current supplying, the nugget Na at the first weld point Pa can grow at a greater growth rate than the nugget Nb at the second weld point Pb.
Thus, by setting the weld period TB for the subsequent negative polarity current to a proper length in view of the weld period TA for the precedent positive polarity current, it is possible at the termination of the weld period TB to give substantially the same growth to both the nugget Na at the first weld point Pa and the nugget Nb at the second weld point Pb.
By alternating the positive current supplying for which the welding current IW flows in a positive direction and the negative current supplying for which the welding current IW flows in a negative direction, it is possible to cancel the influence of Peltier effect to achieve simultaneous joining at the two weld points (Pa and Pb) on the workpieces (W1 and W2) with substantially an even weld strength.
The conventional polarity switching resistance welding power supply apparatus for use in such a series welding is apt to suffer a significant drop in the temperature at the weld points which may occur upon switching of the polarity of current or switching of the polarity of welding current.
By way of example, in the conventional AC waveform inverter power supply apparatus, as seen in FIG. 10, the current supplying is paused till the shutoff of the positive welding current IW after the termination of the inverter switching operation in the positive current supplying (period TA) SO that the inverter switching operation in the negative current supplying (period TB) can start from no-current (IW=0) status to cause the welding current IW to rise in the negative direction. Since the inverter power supply apparatus has a welding transformer intervening between the inverter output terminal and the welding electrode, the inverter tends to face a significantly large load inductance and a substantial time (e.g., 250 xcexcs) is required for the fall of the welding current IW immediately after the stop of the inverter switching, making it difficult to reduce the falltime TH.
Therefore, due to the consumption of substantial time for the fall of the welding current IW and to the delayed start of the next inverse polarity current, the resistance-heating temperature at the weld portions (esp., at and near the weld points Pa and Pb) may possibly remarkably drop for that duration with reduced thermal efficiency, which may adversely affect the weld quality. In particular, this problem was serious in the series welding which is applied to the workpieces in the form of the precision small-sized electronic components.
The present invention was conceived in view of the above problems involved in the prior art. It is therefore the object of the present invention to provide a polarity switching (AC supply) resistance welding power supply apparatus capable of minimizing a substantial current pause time upon polarity switching as far as possible to enhance thermal efficiency of the resistance welding and to improve the weld quality.
In order to attain the above object, according to an aspect of the present invention there is provided a resistance welding power supply apparatus having a pair of welding electrodes through which a welding current flows, the pair of welding electrodes adapted to come into pressure contact with workpieces to resistance weld the workpieces, the resistance welding power supply apparatus comprising a capacitor for storing resistance welding energy in the form of electric charges; charging means arranged to charge the capacitor; first switching means having a first terminal electrically connected to one electrode of the pair of welding electrodes and having a second terminal electrically connected to a first electrode of the capacitor; second switching means having a first terminal electrically connected to the other electrode of the pair of welding electrodes and having a second terminal electrically connected to a second electrode of the capacitor; third switching means having a first terminal electrically connected to the other electrode of the pair of welding electrodes and having a second terminal electrically connected to the first electrode of the capacitor; fourth switching means having a first terminal electrically connected to the one electrode of the pair of welding electrodes and having a second terminal electrically connected to the second electrode of the capacitor; and control means electrically connected to respective control terminals of the first, second, third and fourth switching means, the control means providing a switching control of the first and second switching means while keeping the third and fourth switching means in OFF-state in a first current-supplying mode where the welding current flows through the workpieces in a first direction, the control means providing a switching control of the third and fourth switching means while keeping the first and second switching means in OFF-state in a second current-supplying mode where the welding current flows through the workpieces in a second direction, the control means upon switching between the first and second current-supplying modes initiating a switching control of the following current-supplying mode in the course of decrease of the welding current based on the preceding current-supplying mode.
In the resistance welding power supply apparatus of the present invention, the first to fourth switching means are electrically connected to the welding electrodes without intervention of any welding transformer, so that a small inductance is present on the load side when viewed from the switching means. For this reason, it is easy upon the switching of the current supplying or of the welding current to interrupt or speed up the fall of the welding current in the preceding current supplying, allowing an instantaneous shift to the following current-supplying mode. Thus, by starting switching control for the following current supplying in the middle of the fall of the welding current, it is possible to immediately reverse the polarity of the welding current or the direction of flow of the current and to resume the supply of power to the workpieces.
The resistance welding power supply apparatus of the present invention may further comprise diodes connected in parallel with the first to fourth switching means, each of the diodes being connected to have an opposite polarity of current to that of corresponding one of the first to fourth switching means. Such a configuration makes freewheel current of the welding current rapidly via some of the diodes when the on/off operating switching means are turned off from ON-state in each current-supplying mode.
Preferably, the control means include switching control means which in the first current-supplying mode iteratively turn on/off only one of the first and second switching means at a predetermined frequency while keeping the other of the first and second switching means in ON-state, the switching control means in the second current-supplying mode iteratively turning on/off only one of the third and fourth switching means while keeping the other of the third and fourth switching means in ON-state. Such a configuration also makes freewheel current of the welding current rapidly via one switching element in ON-state when the other on/off operating switching means are turned off from OFF-state.
In the resistance welding power supply apparatus of the present invention, the switching means may each be comprised of a single switching transistor or a plurality of switching transistors that are connected in parallel.